Problem: Let $a$ be the number of positive multiples of $6$ that are less than $30$. Let $b$ be the number of positive integers that are less than $30$, and a multiple of $3$ and a multiple of $2$. Compute $(a - b)^3$.
Let $x$ be a multiple of $6$. Then $x = 6 \cdot n$ for some integer $n$. So $x = 2 \cdot (3n)$ and $x = 3 \cdot (2n)$. This means that $x$ is a multiple of $3$ and $x$ is a multiple of $2$. So multiples of $6$ must be multiples of $2$ and multiples of $3$.

Every number that is a multiple of both 2 and 3 must also be a multiple of the least common multiple of 2 and 3, which is 6.  Hence any number that is a multiple of $3$ and a multiple of $2$ is a multiple of $6$.

We have shown that the numbers that are multiples of $6$ and the numbers that are multiples of $2$ and multiples of $3$ are exactly the same numbers, since any multiple of $6$ is a multiple of $2$ and a multiple of $3$, and any number that is a multiple of $2$ and a multiple of $3$ is a multiple of $6$. So we must have $a = b$. A number minus itself is zero, so our final answer is   $$(a - b)^3 = 0^3 = \boxed{0}.$$